1) Field of the Invention
The present invention relates to a device that has plural printing units each including photosensitive bodies and that forms a color image on a sheet-like medium by overlapping sequentially different colors on the sheet-like medium in the printing units.
2) Description of the Related Art
Electro-photographic printers as color image forming devices, for example, have generally the structure as shown in FIG. 6. The electro-photographic printer 1 consists of a color printing engine 2, sheet cassettes 3 and 4, a sheet feeding unit 5, a sheet ejecting unit 6, a sheet stacker 7, a power supply/control unit 8, and others.
In the electro-photographic printer 1, transfer paper sheets (sheet-like medium) 18 to be printed are stored in the sheet cassettes 3 and 4. At a printing operation, a sheet of transfer paper sheet 18 is sent out of the sheet feeding unit 5 and then guided by means of a conveying roller 23 along a conveying guide (conveying path) 24 to the color printing engine 2. The transfer paper sheet 18 color-printed by the color printing engine 2 (to be described later) is guided by means of the conveying roller 23 via the conveying guide (conveying path) 24 and the sheet ejecting unit 6 and then ejected into the sheet stacker 7.
The power supply/control unit 8 has the function of distributing electric power for the operation of the printer 1 to various portions and controlling the whole operation of the printer 1 including the printing operation of the color printing engine 2.
The printer 1 shown in FIG. 6 includes a double-sided surface mechanism (not shown) that reverses the transfer paper sheet 18 with one surface printed on the side of the sheet ejecting unit 6 to perform a double-sided surface printing on the transfer paper sheet 18 and a conveying guide (conveying path) 24A that again sends the transfer paper sheet 18 reversed by the double-sided surface mechanism to the color printing engine 2.
Generally speaking, the color printing engine 2 for a color printing operation has four printing units 10Y, 10M, 10C, and 10K, a fixing unit 16, an endless transfer belt (a conveying belt such as an electrostatic adsorption belt) 17 made of a resin which conveys the transfer paper sheet 18.
The printing unit 10Y is formed of a photosensitive body (transfer drum, latent image carrier) 11, a pre-charger 12, an optical unit 13, a developing unit 14, and a transfer roller 15 in order to transfer a toner image (developed image) of yellow (Y) on the transfer paper sheet 18. The printing unit 10M is formed of a photosensitive body (transfer drum, latent image carrier) 11, a pre-charger 12, an optical unit 13, a developing unit 14, and a transfer roller 15 in order to transfer a toner image (developed image) of magenta (Z) on the transfer paper sheet 18. The printing unit 10C is formed of a photosensitive body (transfer drum, latent image carrier) 11, a pre-charger 12, an optical unit 13, a developing unit 14, and a transfer roller 15 in order to transfer a toner image (developed image) of cyan (C) on the transfer paper sheet 18. The printing unit 10K is formed of a photosensitive body (transfer drum, latent image carrier) 11, a pre-charger 12, an optical unit 13, a developing unit 14, and a transfer roller 15 in order to transfer a toner image (developed image) of black (B) on the transfer paper sheet 18. The printing units 10Y, 10M, 10C and 10K are arranged nearly in parallel along the transfer belt 17.
The photosensitive body 11 is rotatably driven by means of a drive motor (not shown). The pre-charger 12 charges evenly the surface of the photosensitive body 11. The optical unit 13 projects an image light corresponding to recording information (information regarding print data) on the surface of the photosensitive body 11. The optical unit 13 exposes a pattern corresponding to print data on the surface of the photosensitive body 11 to form an electrostatic latent image.
The developing unit 14 develops an electrostatic latent image formed on the surface of the photosensitive body 11. In fact, the developing process is performed by supplying toner on the surface of the photosensitive body 11 and then forming a toner image (latent image, developing image) being a visual image. The transfer rollers 15 are arranged so as to confront the photosensitive bodies 11 via the transfer belt (or the transfer paper sheet 18) 17. The toner image on the photosensitive body 11 is transferred onto the transfer paper sheet 18 at a transfer point 25 by sandwiching the transfer paper sheet 18 conveyed by the transfer belt 17 at the image transfer point 25 between the transfer roller 15 and the photosensitive body 11.
Further, when the transfer paper sheet 18 on which a toner image of each color is transferred by means of the printing units 10Y, 10M, 10C and 10K is conveyed, the fixing unit 16 fixes the toner image formed on the transfer paper sheet 18 onto the transfer paper sheet 18 by heat, pressure, light, or the like.
The transfer belt 17 is endlessly wound around the drive roller 19, the follower roller 20, and tensioning rollers (tensioners) 21 and 22, and is driven by transmitting the rotational drive force of a drive motor (not shown) from the drive roller 19. The transfer paper sheet 18 which is electrically charged by means of a charger (not shown) is electrostatistically adsorbed on the outer surface (the surface confronting the photosensitive body 11) and then is conveyed sequentially to the printing units 10Y, 10M, 10C and 10K.
In order to arrange in order the ends of transfer paper sheets 18, a resist roller (not shown) is arranged just in front of the image transfer point 25 to the transfer paper sheet 18 in each of the printing units 10Y, 10M, 10C and 10K.
In the electro-photographic printer 1 with the above-mentioned structure shown in FIG. 6, the transfer paper sheet 18 is fed from the sheet cassette 3 or 4 onto the transfer belt 17 of the color printing engine 2 via the sheet feeding unit 5. Then the transfer paper sheet 18 is transferred by means of the transfer belt 17 and then is fed to the fixing unit 16 after passing through the printing units 10Y, 10M, 10C and 10K.
When the transfer paper sheet 18 passes through the printing units 10Y, 10M, 10C and 10K, a toner image of each color (Y, M, C, K) is transferred onto the transfer paper sheet 18. When the transfer paper sheet 18 passes through the fixing unit 16, the toner image is fixed on the transfer paper sheet 18.
When a printing operation is carried out by overlaying sequentially different colors on the transfer paper sheet 18 in the printing units 10Y, 10M, 10C and 10K, a color image is formed on the transfer paper sheet 18.
In the electro-photographic printer 1 above-described, if the drive roller 19 for the transfer belt 17 and drive gears in the drive system (not shown) for driving the drive roller 19 are eccentrically arranged, the velocity of the transfer belt 17 varies. This velocity variation is unavoidable even when the drive roller 19 and the drive system are fabricated accurately.
As shown in FIG. 7, the velocity variation has usually its fixed periodicity in which one revolution of the drive roller 19 is one period. The same velocity variation is repeated every time the drive roller 19 rotates once.
The velocity variation may delicately shift the transfer position of an toner image in each of the printing units 10Y, 10M, 10C and 10K when the photosensitive body 11 and the transfer roller 15 transfer a toner image on the same transfer paper sheet 18 in each of the printing units 10Y, 10M, 10C and 10K. Particularly, in the device that forms a color image by overlapping plural colors, the above-mentioned delicate positional shift causes color separation, thereby deteriorating the print quality of a color image.
It has been proposed that an internal (transfer interval) L between the image transfer points 25 to the transfer paper sheet 18 is set to an integral multiple of the peripheral length .pi.d (where d is a diameter of the drive roller) of the drive roller 19. That is, even when the velocity variation with the periodicity shown in FIG. 7 occurs, the phase of the velocity variation is equalized at the event the same transfer paper sheet 18 reaches the image transfer point in each of the printing units 10Y, 10M, 10C and 10K, with the transfer interval L set to an integral multiple of the peripheral length .pi.d of the drive roller 19. Thus, it can be prevented that the transfer position of the toner image shifts at each image transfer point in the same transfer paper sheet 18.
Strictly speaking, the period of the velocity variation of the transfer roller 19 does not correspond to the peripheral length .pi.d of the drive roller 19, but corresponds to the peripheral length .pi.(d+2t) of the virtual circle 26 considering a thickness t of the transfer belt 17 wound around the drive roller 19. Hence, it can be prevented more surely that the transfer position of a toner image shifts at each image transfer point by setting the transfer interval L to an integral multiple of the peripheral length .pi.(d+2t) of the virtual circle 26. This art is disclosed in Japanese Laid-open Patent Publication (Tokkai-Sho) No. 64-31173.
As described above, in the printer 1 that forms a color image by overlapping four colors in the printing units 10Y, 10M, 10C and 10K, all the printing units 10Y, 10M, 10C and 10K are always operated even in the monochrome printing operation. Here, the photosensitive body 11 in the printing unit with a small use frequency is wasted while the developer used in the printing unit is deteriorated. As a result, the serviceable life of the printing unit with a small use frequency is shortened.
Waste electric power is consumed by operating simultaneously printing units unused. Further, the toner left on the photosensitive body 11 in an unused printing unit stains the transfer paper sheet 18, thereby adversely affecting the print quality.
For the countermeasures, the color image forming device is proposed in which each of the transfer rollers 15 is movably arranged vertically (in the direction separating from the photosensitive body 11 or contacting with the photosensitive body 11) and all the image transfer points are arranged so as to form a slow arc (convexity) when all the printing units 10Y, 10M, 10C and 10K are used. In other words, in each of the printing units 10Y, 10M, 10C and 10K, the photosensitive body 11 and the transfer roller 15 are arranged in parallel so as to have a different height to the horizontal plane while the conveying route (or the transfer belt 17) for the transfer paper sheet 18 is formed like an arc.
In the printing unit for the above-mentioned color image forming device corresponding to an unused color component, the transfer paper sheet 18 can be separated from the photosensitive body 11, together with the transfer belt 17, by moving the transfer roller 15 using a predetermined mechanism in the direction (downward) such that it separates from the photosensitive body 11. Thus, the transfer paper sheet 18 can be set so as not to be contacted with the photosensitive body 11.
With the transfer belt 17 arranged in an arc form, when the transfer belt 17 is separated from the photosensitive body 11 by descending the transfer roller 15 in an unused printing unit, the conveying route between the image transfer points of printing units arranged before and after the unused printing unit changes from an arc into straight line.
Since the conveying route varies from an arc to a straight line when the transfer interval L between the adjacent printing units 10Y, 10M, 10C and 10K is set to an integral multiple of the peripheral length .pi.d or .pi.(d+2t) of the drive roller 19, the transfer interval between the printing units arranged after and before the unused printing unit becomes short, thereby not agreeing with an integral multiple of the peripheral length .pi.d or .pi.(d+2t).
For that reason, when a printing operation is carried out using all the printing units 10Y, 10M, 10C and 10K, the transfer positional shift may not occur. However, when part of the printing units 10Y, 10M, 10C and 10K is used for printing, with the transfer roller 15 of an unused printing unit descended, a transfer positional shift occurs due to a velocity variation. As a result, the resultant color shift causes a deterioration in print quality of a color image.